Microflex circuits are widely used for applications such as integrated circuit (IC) packages, inkjet cartridges, hard disk drives (HDD), medical devices and other advanced flex circuit-based electrical connections. Such connections require electrical static discharge (ESD) protection during product manufacturing and assembly comprising insulating materials (plastic, polymers) and manual operations. ESD concerns are amplified in hard disk drive manufacturing environments and applications, especially for head gimbal assemblies (HGA) using giant magnetorestrictive read (GMR) sensors in the sliders. For current commercial hard disk drive (HDD) product platforms at 40 or 60 gigabit/in2[(GB)/in2], electrostatic discharge protection is required to protect these sensitive read heads. ESD requirements are specified in terms of surface resistivity and tribocharge voltage control on all incoming components and nearby process/assembly workstations. The specifications from many HDD manufacturers indicated surface resistivities should preferably be controlled within a range of from about 104 ohms to about 108 ohms, and maximum surface voltage from tribocharging must be less than 10 Volts (V).
Based on latest HGA technology roadmaps, future HDD programs will be 80 to 100 GB/platter or higher, and will have stricter ESD requirements (tribocharge less than 5 V) for incoming components including flexible circuits for use in flex on suspension (FOS). Microactuator programs, complex slider structures and additional electrical connections and attachment on suspensions will require a good control of ESD as well. Damage caused by ESD accounts for a large portion of the HGA yield losses in the HDD industry, which has a significant impact on industry profits. Therefore, the ability to control ESD is very important to the HDD industry.
In U.S. Ser. No. 09/823,220, the application of a thin, narrow stripe of conductive composition was applied across at least two of the conductive elements of a flexible circuit to reduce the accumulation of static charge and balance the electrical potential across all of the conductive leads. This application also discloses that placing the conductive polymer layer beneath the protective covercoat layer allows the resulting FOS circuit to pass the HDD industry""s strict ionic contamination testing.
However, it has now been discovered that a flexible circuit having a thin conductive polymer coating applied over a portion of a flexible circuit in the non-critical region reduces the measured surface voltage substantially.
The present invention is a flexible circuit incorporating electrostatic discharge (ESD) limiting features and designed for use in the fabrication of hard disk drives for computer applications.
In accordance with one embodiment of the present invention, a thin conductive polymer coating is applied over a portion of a flexible circuit in the non-critical region to reduce the measured surface voltage by enabling the flow of tribocharging induced charges on the coated surface allowing faster discharge from the affected area when contacted by a conducting, grounded probe. The conductive layer limits the rate of discharge due to its inherent resistivity. If the coating is in contact with a conductive polymer stripe, the built-up charges can also be discharged through probes contacting the coated, non-critical surface. The resulting circuits have added ESD protection because the conductive surface will enable discharge of any charges developed on its surface by starting the discharge as soon as they are created, thus lowering the peak voltage on the surface by enabling and controlling voltage level decay. Since the available energy to damage the GMR head is proportional to the voltage level when the energy is coupled through the sensitive GMR sensor, the total energy is reduced.
The thin conductive film will significantly reduce surface resistivity of the insulating materials from about 1011 ohms to about 1014 ohms to a range of about 104 ohms to about 108 ohms.
As used herein, these terms have the following meanings:
1. The term xe2x80x9cnon-critical regionxe2x80x9d means the region of the circuit between the actuator flex bond region and the gimbal region of the circuit. This is the area normally used for part handing during the assembly process.
2. The terms xe2x80x9cinterlever materialxe2x80x9d and xe2x80x9cinterlever layerxe2x80x9d mean a layer of a material such as a release liner placed under the circuit to transfer the flex circuit web into the printing machine through splicing.